The present invention relates to the field of seismic data acquisition operations. More particularly, the present invention relates to a unique system for synchronizing seismic subsystems through independent module timing.
Seismic data acquisition requires the coordination of numerous system components. In marine seismic operations, seismic vessels tow air guns through water to generate seismic energy sources. Acoustic waves from the air guns travel downwardly into the water and underlying geologic structures and are reflected by interfaces between the geologic structures. The reflected signal impulses return to the surface and are detected with sensors towed behind the seismic vessel.
Seismic signal processing requires time and position correlation between the seismic energy sources and the reflected signals. The length of time between energy sources and the reflected signals depends on factors such as the water depth, subsurface elevation of the reflective interface, and subsurface geology. Synchronization of the sources and reflected signals are necessary to the accurate evaluation of the data. Synchronization is necessary for the position of the individual components, the shot times, the shot recording times, and sampling times within the seismic data recording system. Additionally, synchronization is relevant to quality control procedures for testing the accuracy in the time domain and for testing event real times as compared with ideal times.
Numerous timing error sources affect the accuracy of the entire seismic survey system. The reduction of error sources is particularly important in three-dimensional seismic collection and in processing systems having data loads and error sources not found in conventional two-dimensional seismic technology. In conventional synchronization systems, these error sources are reduced by timing all events to a single clock which provides an absolute time reference.
In a marine seismic system, synchronization depends on the absolute time, vessel speed and subsystem position data. The seismic vessel navigation system determines the location where the air guns generate acoustic source energy. Each reflected pulse is detected by hydrophone data recording subsystems, and signals are transmitted through hard lines or through radio transmission. In large arrays, a central controller broadcasts an absolute time signal to each of the subsystems, and the acquired data is transmitted back to the central controller. Such continuous data transmission requires significant and generally continuous bandwidth capabilities.
Conventional event synchronization systems manage seismic event times as discrete events referenced to an absolute time scale. All events within the seismic survey system are actuated by trigger pulses from a navigation subsystem initiating pulse, and such operating systems reduce system synchronization errors by broadcasting an absolute time signal to internal clocks in each subsystem. The navigation system predicts the firing position for a source at a selected event time, the reference time is broadcast from a central controller to all survey subsystems before the selected event time, and the individual subsystems generate event pulses based on the pre-programmed times. In this system architecture, each subsystem depends upon the accuracy of the synchronized time standard. Accordingly, systematic errors in the transfer and triggering of the pulses causes delays and inaccuracies throughout the entire system.
Numerous techniques have been developed to address the synchronization and control of a seismic gathering system. U.S. Pat. No. 3,144,651 to Savit et al. (1964) disclosed an apparatus for recording seismic input signals. The recording cycle was commenced by a first event such as the rotation of a time recording drum. A second event corresponding to a defined "fiducial time (T=0)" generated a time break signal for disconnecting the time recording drum. In U.S. Pat. No. 3,704,444 to Schmit (1972), a program sequencer operated a timing clock, and U.S. Pat. No. 3,744,019 to Schmitt (1973) disclosed an absolute time clock for coordinating the field collection of seismic signals.
In U.S. Pat. No. 4,041,443 to Thigpen (1977), a clock engaged with a controller synchronized operations of seismic processing components, and the controller provided selected time zones for enabling different sample rates. In U.S. Pat. No. 4,042,906 to Ezell (1977), seismic data was transmitted in real time from individual slave stations to a master station having an absolute time clock oscillator and interval counter. The shot instant and the range of individual slave stations was transmitted to the master station, and a sequencing circuit was provided by clock pulse inputs from the clock oscillator.
In U.S. Pat. No. 4,092,629 to Siems et al. (1978), a repeater network transmitted a local self clocking phase-encoded data word through a data link to a central station. Subsequently, the repeater network received, regenerated and retransmitted self clocking phase-encoded data words from down-link transceiver units. A multiplexer initiated a scan cycle which was interruptable by resetting the multiplexer after signals from a preselected number of input channels was sampled. In U.S. Pat. No. 4,152,691 to Ward (1979), a master control and collation system provided a time-zero signal for correlating activation of a source and a time base for data acquisition units, and provided a time-check signal at the end of the acquisition unit recordings. In U.S. Pat. No. 4,224,474 to Savit (1980), a series of recording signals were initiated at desired intervals, and a predetermined number of recording channels were identified within a selected block. Consecutive recording cycles within a block were assigned unique corresponding addresses.
In U.S. Pat. No. 4,549,285 to Fleurance et al. (1985), data correlations were conducted in real time with a pilot signal. In U.S. Pat. No. 4,589,100 to Savit (1986), a central station included a master clock for firing a sound source. Remote seismic data recording units each included a separate clock for performing functions such as selectively deactivating the remote unit to conserve power. In U.S. Pat. No. 4,686,474 to Olsen et al. (1987), system synchronization was provided by a clock within a master computer which triggered local clocks for synchronizing geophysical and positioning data gathering. U.S. Pat. No. 5,058,080 to Siems et al. (1991) performed system synchronization by the continuous transmission of a command signal having a clocking signal.
U.S. Pat. No. 5,548,562 to Helgerud et al. (1996) describes a pulse synchronized system having a master controller for anticipating an event time and for generating a pulse or command at the correct time. A receiver obtained signals from a satellite navigation system such as Global Positioning System (GPS) based on an operable time standard such as Universal Time Coordinated (UTC). Such time standard was then supplied to selected subsystems in the seismic survey system, and the subsystems generated an event at the designated clock time. A pulse or command was required for each desired event, and the clock continuously provided signals to the system components.
As described above, conventional synchronization systems require a master clock which provides an absolute time standard for execution of subsystem events. Such systems also require continuous two-way data communication and coordination between the master controller and remote, individual subsystems. This two-way communication requirement consumes available communication links and requires accurate subsystem clocks correlated to the master clock.
The system errors and processing requirements of conventional systems demonstrate a need for an improved seismic synchronization system which can communicate more data within the available communications bandwidth. The system should be reliable and should efficiently transmit seismic event data within acceptable accuracy limits.